It is well known that the heat transfer co-efficient for boiling a liquid may be improved by coating the heat transfer surface of a heat exchanger with a matrix of small heat-conductive particles which produce a network of linked re-entrant cavities. The cavities act as nucleation centres for the production of bubbles of vapour.
So called "high flux" (trademark) tubing having a high co-efficient of heat transfer is available but its production is complex and the tubing is correspondingly expensive. High flux tubing is generally produced by coating a tube with petroleum jelly and depositing particles of a cupro-nickel powder onto the tube so that the particles become embedded in the petroleum jelly. The cupro-nickel powder is made up of two components having different melting points. The coated tube is then heated in a furnace to a temperature such that one of the components just melts and adheres the other non-melted powder to the tube. In this way, a matrix of metal particles firmly adhered to the tube is produced.
There have in the past been various attempts to provide a high heat transfer surface by plasma spraying liquified metal onto a substrate, but generally speaking, results have not been consistent.
European Patent application 88307468.4 describes a method of coating heat transfer surfaces by spraying a particulate mixture of metal and a plastics material onto a thermally conductive surface to form a coating comprising particles of plastics material embedded in metal, and heating to a temperature sufficient to volatilise the plastics material, thereby forming pores in the metal coating.
Our earlier European Patent application 91310265.3 describes the production of a surface coating comprising porous carbon particles embedded in a matrix of metal particles produced by plasma spraying a mixture of particles of metal and carbon onto a substrate in an inert gas atmosphere.
U.S. Pat. No. 3,384,154 describes a method of coating heat transfer surfaces with small metallic particles which are subsequently sintered onto the substrate material.
U.S. Pat. No. 4,753,849 describes a process of coating an evaporator tube which involves arc spraying two dissimilar metals onto the tube and then etching out one of the metals.
European application 80101983.7 describes the production of a porous boiling surface by spraying liquified aluminium onto a substrate under inert gas according to certain specified conditions.